Recently, reinforcement of a blade cascade performance of an axial-flow fluid machine including a steam turbine, a gas turbine and the like has been required to be re-examined by reducing a secondary flow loss of a secondary flow of the working fluid, for example.
The secondary flow loss of the secondary flow may cause great loss as serious as the profile loss defined by the configuration of the blade type.
The secondary loss is considered to be caused by the mechanism to be described hereinafter.
FIG. 27 is a conceptual view that explains the mechanism that causes the secondary flow, which is cited from a reference titled “Fundamentals and practice of a gas turbine” (by Miwa, Published on Mar. 18, 1989, Seibundo Shoten, p. 119).
FIG. 27 is an exemplary conceptual view of a turbine nozzle when seen from a rear edge of the blade body.
The working fluid, for example, steam flowing into a flow passage 4 formed between the blade cascade including adjacent blade bodies 1a and 1b, and wall surfaces 3a and 3b each supporting tip portions and root portions of the respective blade bodies 1a and 1b is curved like an arc as it passes through the flow passage 4 so as to further flow into the next blade cascade.
When the working fluid passes through the flow passage 4, a centrifugal force is generated in the direction from a back side 5 of the blade body 1b to a front side 6 of the blade body 1a adjacent thereto. The static pressure at the front side 6 of the blade body 1a is relatively high to make a balance with the centrifugal force. Meanwhile, the static pressure at the back side 5 of the other blade body 1b is relatively low as the flow rate of the working fluid is high.
In this case, a pressure gradient occurs in the flow passage 4 from the front side 6 of the blade body 1a to the back side 5 of the other blade body 1b adjacent thereto. The pressure gradient also occurs around boundary layers at the root portions and the tip portions of the blade bodies 1a and 1b, respectively.
Because the flow rate of the working fluid at the boundary layer is low and the centrifugal force thereat is small, it is not capable of resisting against the pressure gradient from the front side 6 of the blade body 1a to the back side 5 of the adjacent blade body 1b. This may generate the secondary flow of the working fluid from the front side 6 to the back side 5 of the blade body 1b. The secondary flow partially contains horseshoe vortexes (horseshoe-like vortex) 8a and 8b generated upon collision of the working fluid against front edges 7a and 7b of the blade bodies 1a and 1b, respectively.
Each of the horseshoe vortexes 8a and 8b flows across the width of the flow passage 4 toward the back side 5 of the adjacent blade body 1b in the form of a passage vortex 9, which swirls up the boundary layer while being interfered with a corner vortex 10 at the back side 5 of the adjacent blade body 1b. The resultant vortex becomes the secondary flow vortex.
The secondary flow vortex disturbs the main flow (drive fluid) as the cause of the reduction in the blade cascade efficiency.
FIG. 28 is a graph representing a loss derived from the 3-D (three-dimensional) numerical data fluid analysis as to how the secondary flow of the working fluid influences the reduction in the blade cascade efficiency. The vertical axis of the graph represents the height of the blade body, and the horizontal axis of the graph represents a full pressure, respectively.
Observing the 3-D numerical data fluid analysis, it is recognized that the secondary flow from the front side 6 of the blade body 1a to the back side 5 of the adjacent blade body 1b occurs at the root and the tip sides of the blade, respectively.
As a result of further observation of the 3-D numerical data fluid analysis, it is recognized that the full pressure loss becomes considerably high in the area (areas A and B in FIG. 28) where the secondary flow vortex caused by the passage vortexes 9a and 9b swirling around the adjacent blade body 1b meet the horseshoe vortexes 8a and 8b generated through collision against the front edges 7a and 7b of the blade bodies 1a and 1b to flow along the back side 5.
Various types of technology have been disclosed in Publications of Japanese Patent Application Laid-Open Publication Nos. HEI 1-106903, HEI 4-124406, 9-112203, 2000-230403 with respect to the development of the process for suppressing the reduction in the efficiency of the blade cascade caused by the secondary flow based on the investigation with respect to the mechanism thereof.
The U.S. Patent Publication No. 6,419,446 discloses the process for reducing the secondary flow loss by providing a cusp-like protruding portion in a stagnation area around portions defined by the front edges 7a and 7b of the blade bodies 1a and 1b and the wall surfaces 3a and 3b, respectively to diminish the strength of the passage vortexes 9a and 9b. 
The reference titled “Controlling Secondary-Flow Structure by Leading-Edge Airfoil Fillet and Inlet Swirl to Reduce Aero-dynamic Loss and Surface Heat Transfers” (Proceedings of ASME TURBO EXPO 2002, Jun. 3-6, 2002 Amsterdam the Netherlands, GT-2002-30529) reports that the flow rate of the working fluid flowing to the cusp-like protruding portion provided in the stagnation area around the portion defined by the front edges 7a and 7b of the blade bodies 1a and 1b and the wall surfaces 3a and 3b, respectively, is accelerated, and the thus accelerated flow of the working fluid serves to eliminate the horseshoe vortexes 8a and 8b so as to diminish the strength of the passage vortexes 9a and 9b. 
The reference describes an effect derived from the cusp-like rounded protruding portion. As the cusp-like protruding portion has a function in forcing the horseshoe vortexes 8a and 8b away from the front edges 7a and 7b of the blade bodies 1a and 1b, the strength of the passage vortexes 9a and 9b may be diminished, thus reducing the blade cascade loss. However, it also reports that the aforementioned effect may be obtained on the assumption that an edge line (parting line) of the rounded cusp-like protruding portion is required to coincide with a stagnation point (at which the working fluid collides against the front edges of the blade body) of the working fluid.
As the flow rate of the working fluid flowing into the blade bodies 1a and 1b may vary with the load (output), it is difficult to control an incident angle of the working fluid especially at a time of the start-up operation, the partial load operation, and the like.
There has been a demand to further broaden the scope of the technology disclosed in the U.S. Patent Publication No. 6,419,446 as described above for the purpose of providing the turbine blade cascade capable of reducing the secondary flow loss irrespective of the fluctuation in the flow rate of the working fluid, and discord between the edge line of the rounded cusp-like protruding portion and the stagnation point of the working fluid.